The invention concerns a nuclear magnetic resonance (NMR) resonator comprising an inductive section and a capacitive section, wherein the inductive section is band-shaped and surrounds a substantially cylindrical volume under investigation, wherein the capacitive section is formed from one or several discrete capacitor(s), and wherein the ends of the band-shaped inductive section are connected through the one or several capacitor(s) of the capacitive section.
An NMR resonator of this type is disclosed e.g. in DE 42 23 909 A1.
Nuclear magnetic resonance spectroscopy is an effective method of instrumental analysis. RF (radio frequency) pulses are irradiated into a sample that is disposed in a static magnetic field, and the RF reaction of the sample is measured. The RF reaction provides information about the properties of the sample. NMR resonators are used to irradiate RF pulses and to read out the RF sample reaction. The resonance frequency of the NMR resonators must thereby be adjusted to the respective measurement.
For so-called high-resolution NMR, very strong static magnetic fields are used, which are usually generated by helium-cooled superconducting coil systems. High-resolution NMR produces detailed, complete NMR spectra that provide an exact analysis of the chemical composition of a sample, e.g. identification and determination of the amount of different binding types. The resonance frequencies with respect to protons are typically within a range of some hundreds of mega hertz (MHz).
However, for so-called low-resolution NMR (also called TD-NMR, TD=time domain), weaker static magnetic fields are used, which can be generated with permanent magnets. Low-resolution NMR enables determination of individual properties of samples with simple and inexpensive apparatus, e.g. the fraction of a certain substance in a sample. The resonance frequencies with respect to protons are typically within a range of some tens of MHz.
An NMR resonator substantially represents an oscillating circuit that comprises a capacitive section (with capacitance C) and an inductive section (with inductance L). The resonance frequency f of the NMR resonator is thereby determined in accordance with the formula f=1/(2π√{square root over (LC)}). The resonance frequency of an NMR resonator can therefore be adjusted through suitable selection of L and C.
FIG. 4 of the above-mentioned document DE 42 23 909 A1 shows an NMR resonator with a C-shaped bent slotted metal sheet (split ring), the ends of which are connected to one or several capacitor(s). NMR resonators of this type and similar types are currently also used in low-resolution NMR, e.g. for measuring protons in a 40 MHz magnet. The resonance frequency of the above-mentioned resonator type can be adjusted through suitable selection of the capacitance C (e.g. of the capacitor(s)).
However, some applications of low-resolution NMR require particularly low resonance frequencies. An NMR resonator with a resonance frequency of approximately 10 MHz is e.g. required for measuring C13 atoms in the above-mentioned 40 MHz magnet.
If the capacitance C of the resonator type disclosed in DE 42 23 909 A1 is increased in order to reduce the resonance frequency to a range of about 10 MHz, the quality of the NMR measuring results deteriorates considerably.
A further NMR resonator with a spirally bent, band-shaped section is disclosed e.g. in DE 10 2005 024 773 B3 (FIG. 2). The inner end of the band-shaped section is contacted via an axial lead-through conductor. However, this NMR resonator is mechanically unstable and is very difficult to produce with precision. The usable volume under investigation is also quite small due to the spiral shape.
There are further NMR resonators which comprise a solenoid coil with numerous windings as the inductive section. NMR resonators of this type require a support body inside the solenoid coil in order to stabilize the conductor windings, in particular, with respect to acoustic vibrations. The support body considerably reduces the usable volume under investigation. This type of resonator is also quite expensive to produce.
It is therefore the underlying purpose of the invention to provide an NMR resonator for low-resolution NMR, which has a simple construction and provides NMR measurements of improved quality with low resonance frequencies.